1. Field of the Invention
The present invention relates to a scanning electron microscope for observing and measuring a fine pattern formed on a wafer in the production process of integrated circuits, and to an electron optic column used therein.
2. Related Background Art
Scanning electron microscopes for observation of semiconductor wafers, for example represented by the dimension measurement system (dimension SEM) using an electron beam (EB), are increasing their size year by year with increasing diameter of the wafer. In the basic structure of an EB dimension measurement system which is generally used at present, an electron optic column (column) is mounted on a sample chamber enclosing a wafer.
The electron optic column comprises an electron gun enclosed in an electron gun chamber, an ion pump for keeping the electron gun chamber at high vacuum, a condenser lens and an objective lens for converging an electron beam emitted from the electron gun into a fine beam, and a secondary electron detector.
The secondary electron detector could be placed outside the column but is often constructed recently as incorporated into the column above or right below the objective lens to improve the detection efficiency for observation at higher resolution.
There are an XY stage (or XYZ stage if oblique observation is necessary) provided inside the sample chamber and a wafer placed on the XY stage. Motor drive controls movement of the XY stage as will be described below. In an EB dimension measurement system, the upper stage will be referred to as a Y-axis stage and the lower stage as an X-axis stage. The operational theory and construction will be described below.
The X-axis stage is held by an X-axis cross roller guide and slides to move in the X direction. In more detail, the X-axis cross roller guide is fixed on a stationary platen and the X-axis stage is given a driving force in the X direction by an X-axis pulse motor and an X-axis ball screw fixed to the stationary platen so as to slide to move on the X-axis cross roller guide.
The Y-axis stage is mounted on a Y-axis cross roller guide and is moved in the Y direction by a Y-axis pulse motor and a Y-axis ball screw not shown. The movement of the XY stage is determined by a combination of these operations.
In order to observe the entire surface of a wafer, the XY stage requires a movement space which is at least a double of wafer size along the both X and Y axes. In other words, the XY stage needs a movement space of at least 40 cm.times.40 cm for 8 inch wafer (in diameter of 20.32 cm). In actual applications a transfer mechanism of driving force is necessary in addition to the above arrangement, so that the necessary space becomes larger. Although the weight of the XY stage differs considerably depending upon demanded accuracy, the XY stage alone for observation of 8 inch wafers, for movement only in the X, Y directions and with stop accuracy of .+-.2-3 .mu.m has a weight of about 20 Kg.
As the size of the wafer increases, a pattern becomes finer. Then, the movement accuracy and the stop accuracy required of the XY stage must be enhanced, and the movement speed must be increased. Namely, as the wafer size increases, a movement time from edge to edge on the wafer increases if the movement speed is unchanged. To satisfy these requirements, the structure of the XY stage becomes more and more complex, resulting in a size increase of the XY stage. The size increase in turn results in an increase of the entire floor space (floor area) of the scanning electron microscope. The scanning electron microscopes are actually used under the essential condition of inprocess control (to keep the wafer within a clean room). Considering the scanning electron microscope used as set in clean room, the size increase of the scanning electron microscope will raise a serious problem. Nevertheless, the size of the scanning electron microscope rather tends to increase because of the reasons as described above.
The basic reason why the size of scanning electron microscope, specifically the area of the floor space, increases with an increase in diameter of the wafer is that the entire surface of the wafer is to be observed by moving the XY stage while keeping the column fixed. As far as this method is maintained, the stroke of the XY stage increases at least by a double of diameter increase as the diameter of the wafer increases (an area increase becomes four times larger than the diameter increase).
One of the major factors to employ such apparatus arrangement is that the conventional columns are large in shape and weight. Describing this point in more detail, there are the following causes: (1) electromagnets are heavy in the conventional columns using magnetic field type lenses; (2) an evacuation pump for obtaining high vacuum is heavy.
The conventional scanning electron microscopes (SEM) employ the magnetic field type lenses. A magnetic field type lens converges an electron beam by the lens effect of a magnetic field produced by an electromagnet, which needs a magnetic yoke for shaping the magnetic field from its theoretical structure. In the normal SEM currently used, the lens construction includes three-stage focusing lenses (a stage of objective lens and two stages of condenser lenses). Additional lenses should be prepared for alignment correction, for astigmatism correction and for beam scanning in addition to the lenses for focusing. Accordingly, the total weight including the housing of the column will be at least 20 kg-30 Kg.
A specific feature of the magnetic field type lenses is excellent aberration property on average in a wide energy range (approximately 5 keV-20 keV in normal applications of SEM). Also, the magnetic field type lenses have been used in actual applications since the electron microscopes. Thus, the magnetic field type lenses are widely used for these reasons. However, if the utility is limited to the observation of the wafer, the circumstances would change. Samples of semiconductor wafer require observation with electron beam of far lower energy than in a conventional procedure to avoid degradation of property by irradiation of electron beam (irradiation damage) or to relieve charge-up in the observation object, because most of observed objects are insulators. Specifically, the observation is carried out with an electron beam of low energy of 0.5 keV14 1.5 keV.
A short focus and high excitation type lens is necessary to obtain an excellent aberration property in a low energy region if the magnetic field type lens is employed. Then, a strong magnetic field is necessarily produced. The excitation current of the objective lens must be increased in particular. Thus, the objective lens inevitably increases its size and weight. High ampere-turns are necessary to produce a strong magnetic field, which would cause a problem of heating by the current and therefore require water cooling for lowering the temperature of lenses.
Next described is a problem in the vacuum evacuation system. The problem mainly arises from a vacuum pump for evacuating the electron gun chamber. As described before, focusing of low energy electrons is necessary for observation of semiconductor samples. As the electron energy becomes lower, the influence of aberration of a lens, specifically of the objective lens, for example the influence of chromatic aberration, increases, making the focusing of the beam more difficult as a result. To compensate it, an electron gun of high brightness type is employed.
A field emission type electron gun has high brightness but little energy dispersion of emitted electrons. Therefore, it is most suitable for this purpose, but needs a high vacuum in the electron gun chamber to ensure a stable operation, which is a drawback. Specifically, a cold-cathode type electron gun (CFE) needs a high vacuum of not less than 10.sup.-10 Torr and a thermal field emission type electron gun (TFE) needs a high vacuum of not less than 10.sup.-9 Torr. For this purpose, the conventional procedure employs an arrangement that only the electron gun is independently evacuated by an ion pump.
As detailed above, the following points were a hindrance to size reduction and weight reduction of an apparatus for observing large samples such as wafers.
(1) The XY stage on which a large sample (wafer) is mounted must be moved while the column is kept fixed, which theoretically increases the scale of the sample chamber.
(2) The column must be fixed on the sample chamber, because the column is large and heavy.
(3) The column is large and heavy, because the column includes the magnetic field type lenses.